Wireless handheld or portable devices typically operate one or more cellular communication standards, and/or wireless connectivity standards, and/or broadcast standards, each standard being allocated in one or more frequency bands, and said frequency bands being contained within one or more regions of the electromagnetic spectrum.
For that purpose, a space within the wireless handheld or portable device is usually dedicated to the integration of a radiating system. The radiating system is, however, expected to be small in order to occupy as little space as possible within the device, which then allows for smaller devices, or for the addition of more specific equipment and functionality into the device. At the same time, it is sometimes required for the radiating system to be flat since this allows for slim devices or in particular, for devices which have two parts that can be shifted or twisted against each other.
Many of the demands for wireless handheld or portable devices also translate to specific demands for the radiating systems thereof.
A typical wireless handheld or portable device must include a radiating system capable of operating in one or more frequency bands of the electromagnetic spectrum with good radioelectric performance (such as for example in terms of input impedance level, impedance bandwidth, gain, efficiency, or radiation pattern). Moreover, the integration of the radiating system within the wireless handheld or portable device must be correct to ensure that the wireless handheld or portable device itself attains a good radioelectric performance (such as for example in terms of radiated power, received power, sensitivity, or SAR).
This is even more critical in the case in which the wireless handheld device is a multifunctional wireless device. Commonly-owned patent applications WO2008/009391 and US2008/0018543 describe a multifunctional wireless device. The entire disclosure of said application numbers WO2008/009391 and US2008/0018543 are hereby incorporated by reference.
For a good wireless connection, high gain and efficiency are further required. Other more common design demands for radiating systems are the voltage standing wave ratio (VSWR) and the impedance which is supposed to be about 50 ohms.
Other demands for radiating systems for wireless handheld or portable devices are low cost and a low specific absorption rate (SAR).
Furthermore, a radiating system has to be integrated into a device or in other words a wireless handheld or portable device has to be constructed such that an appropriate radiating system may be integrated therein which puts additional constraints by consideration of the mechanical fit, the electrical fit and the assembly fit.
Of further importance, usually, is the robustness of the radiating system which means that the radiating system does not change its properties upon smaller shocks to the device.
A radiating system for a wireless handheld or portable device typically includes a radiating structure comprising an antenna element which operates in combination with a ground plane layer providing a determined radioelectric performance in one or more frequency regions of the electromagnetic spectrum. This is illustrated in FIG. 23, in which it is shown a conventional radiating structure 2300 comprising an antenna element 2301 and a ground plane layer 2302. Typically, the antenna element has a dimension close to an integer multiple of a quarter of the wavelength at a frequency of operation of the radiating structure, so that the antenna element is at resonance or substantially close to resonance at said frequency and a radiation mode is excited on said antenna element. It is important to stress that the relationship between the operating wavelength and the size of the antenna is due to a well-known principle that an antenna needs to keep a minimum proportion with respect to such operating wavelength to radiate efficiently. Therefore, it is the conventional wisdom that an antenna which is much smaller than the wavelength would radiate quite inefficiently, and in the limit, would not radiate at all. The fundamental limitations of small antennas where first established by Chu and Wheeler in the 1940's; who described that a small antenna inherently suffered of a reduced bandwidth and eventually a decreased radiation efficiency.
In some cases, the antenna element acting in cooperation with the ground plane does not attain sufficient impedance bandwidth as for covering multiple communication standards and a matching network must be added between the antenna element and the input/output port in order to increase said impedance bandwidth. Some inconveniences of adding matching networks in multiband radiating systems mainly rely on the fact that usually the proper values to match a particular frequency band not necessary coincide with those required to match another frequency band. This inconvenience further exacerbates when the frequency bands to match are allocated at separate frequency regions of the electromagnetic spectrum.
In addition, antenna elements operating in multiple frequency bands allocated at different regions of the electromagnetic spectrum usually presents a complex geometry and considerable dimensions, mainly due to the fact that antenna performance is highly related to the electrical dimensions of the antenna element.
A further problem associated to the integration of the radiating structure, and in particular to the integration of the antenna element, in a wireless device is that the volume dedicated for such an integration has continuously shrunk with the appearance of new smaller and/or thinner form factors for wireless devices, and with the increasing convergence of different functionality in a same wireless device.
Some techniques to miniaturize and/or optimize the multiband behavior of an antenna element have been described in the prior art. However the radiating structures described therein still rely on exciting a radiation mode on the antenna element for each one of the frequency bands of operation. This fact leads to complex antenna elements that usually are very sensitive to external effects (such as for instance the presence of plastic or dielectric covers that surround the wireless device), to components of the wireless device (such as for instance, but not limited to, a speaker, a microphone, a connector, a display, a shield can, a vibrating module, a battery, or an electronic module or subsystem) placed either in the vicinity of, or even underneath, the radiating element, and/or to the presence of the user of the wireless device. A multiband antenna system is sensitive to any of the above mentioned aspects because they may alter the electromagnetic coupling between the different geometrical portions of the radiating element, which usually translates into detuning effects, degradation of the radioelectric performance of the antenna system and/or the radioelectric performance wireless device, and/or greater interaction with the user (such as an increased level of SAR).
For example, commonly-owned co-pending patent application US2007/0152886 describes a new family of antennas based on the geometry of space-filling curves. Also, commonly-owned co-pending patent application US2008/0042909 relates to a new family of antennas, referred to as multilevel antennas, formed by an electromagnetic grouping of similar geometrical elements. The entire disclosures of the aforesaid application numbers US2007/0152886 and US2008/0042909 are hereby incorporated by reference.
In this sense, a radiating system not requiring a complex antenna formed by multiple arms, slots, apertures and/or openings such as the one described in the present invention is preferable in some embodiments in order to minimize such undesired external effects.
Some other attempts have focused on antenna elements not requiring a complex geometry while still providing some degree of miniaturization by using an antenna element that is not resonant in the one or more frequency ranges of operation of the wireless device.
For example, WO2007/128340 discloses a wireless portable device comprising a non-resonant antenna element for receiving broadcast signals (such as, for instance, DVB-H, DMB, T-DMB or FM). The wireless portable device further comprises a ground plane layer that is used in combination with said antenna element. Although the antenna element has a first resonant frequency above the frequency range of operation of the wireless device, the antenna element is still the main responsible for the radiation process and for the electromagnetic performance of the wireless device. This is clear from the fact that no radiation mode can be excited on the ground plane layer because the ground plane layer is electrically short at the frequencies of operation (i.e., its dimensions are much smaller than the wavelength). For this kind of non-resonant antenna elements, a matching circuitry is added for matching the antenna to acceptable level of VSWR which in this particular case can be around VSWR≦6, which is only acceptable for reception of electromagnetic wave signals but not enough for allowing their transmission.
With such limitations, while the performance of the wireless portable device may be sufficient for reception of electromagnetic wave signals (such as those of a broadcast service), the antenna element could not provide an adequate performance (for example, in terms of input return losses or gain) for a communication standard requiring also the transmission of electromagnetic wave signals.
Commonly-owned patent application WO2008/119699 describes a wireless handheld or portable device comprising a radiating system capable of operating in two frequency regions. The radiating system comprises an antenna element having a resonant frequency outside said two frequency regions, and a ground plane layer. In this wireless device, while the ground plane layer contributes to enhance the electromagnetic performance of the radiating system in the two frequency regions of operation, it is still necessary to excite a radiation mode on the antenna element. In fact, the radiating system relies on the relationship between a resonant frequency of the antenna element and a resonant frequency of the ground plane layer in order for the radiating system to operate properly in said two frequency regions.
Nevertheless, the solution still relies on a complex matching network including resonators and filters for each frequency region of operation.
The entire disclosure of the aforesaid application number WO2008/119699 is hereby incorporated by reference.
Other attempts for covering several frequency bands allocated in a particular frequency region of the electromagnetic spectrum rely on the use of antenna elements distributed along the ground plane of a wireless handheld or portable device as disclosed in a commonly-owned patent application WO2007/141187. Each one of the antenna elements of said distributed antenna system resonates or substantially resonates at a frequency within a first frequency region of the electromagnetic spectrum, thus providing redundancy to the radiating system. Said redundancy allows increasing the robustness to human loading effects.
Another limitation of current wireless handheld or portable devices relates to the fact that the design and integration of an antenna element for a radiating structure in a wireless device is typically customized for each device. Different form factors or platforms, or a different distribution of the functional blocks of the device will force to redesign the antenna element and its integration inside the device almost from scratch.
For at least the above reasons, wireless device manufacturers regard the volume dedicated to the integration of the radiating structure, and in particular the antenna element, as being a toll to pay in order to provide wireless capabilities to the handheld or portable device.
In order to reduce as much as possible the volume occupied into the wireless handheld or portable device, recent trends in handset antenna design are oriented to maximize the contribution of the ground plane to the radiation process by using non-resonant elements. However, non-resonant elements usually are forced to include a complex radiofrequency system. Thus, the challenge of these techniques mainly relies on said complexity (combination of inductors, capacitors, and transmission lines), which is required to satisfy impedance bandwidth and efficiency specifications.
Commonly owned patent applications, WO2010015365 and WO2010/015364 are intended for solving some of the aforementioned drawbacks. Namely, they describe a wireless handheld or portable device comprising a radiating system including a radiating structure and a radiofrequency system. The radiating structure is formed by a ground plane layer presenting suitable dimensions as for supporting at least one efficient radiation mode and at least one radiation booster capable of coupling electromagnetic energy to said ground plane layer. The radiation booster is not resonant in any of the frequency regions of operation and consequently a radiofrequency system is used to properly match the radiating structure to the desired frequency bands of operation.
More particularly, in WO2010/015364 each radiation booster is intended for providing operation in a particular frequency region. Thus, the radiofrequency system is designed in such a way that the first internal port associated to the first radiation booster is highly isolated from the second internal port associated to a second radiation booster. Said radiofrequency system usually comprises a matching network including resonators for each one of the frequency regions of operation and a set of filters for each one of the frequency regions of operation. Thus, said radiofrequency system requires multiple stages and the performance of the radiating systems in terms of efficiency may be affected by the additional losses of the components.
A radiation booster should not be confused with a radiating element. Being much smaller than the operating wavelength of the system, the radiation booster alone would be incapable to transmit or receive electromagnetic signals within such operating wavelength. Therefore, a radiation booster can not be considered on its own an antenna or a radiating element.
Another technique, as disclosed in U.S. Pat. No. 7,274,340, is based on the use of non-resonant elements where the impedance matching is provided through the addition of two matching circuits. The two non-resonant elements are arranged in such a manner that they provide coupling to the ground plane. Despite the use of two non-resonant elements, the size of the element for the low band is significantly large, being 1/9.3 times the free-space wavelength of the lowest frequency for the low frequency band. Due to such size, the low band element would be a resonant element at the high band. The size of the low band element undesirably contributes to increase the printed circuit board (PCB) space required by the antenna module. In fact, such radiating system is still about the size of a conventional internal antenna inside a handset, therefore the overall radiating system does not provide a significant space advantage compared to the existing alternative solutions.
Therefore, a wireless device including small antenna elements or even not requiring an antenna element together with a simplified radiofrequency system would be advantageous to make simpler the integration of the radiating structure into the wireless handheld or portable device. The volume freed up by the absence of a large and complex antenna element would enable smaller and/or thinner devices, or even to adopt radically new form factors (such as for instance elastic, stretchable and/or foldable devices) which are not feasible today due to the presence of an antenna element featured by a considerable volume. Furthermore, by eliminating precisely the element that requires customization, a standard solution is obtained which only requires minor adjustments to be implemented in different wireless devices.